Technical Field
The present invention relates to a technology pertaining to diagnosis of abnormality of an exhaust gas purification device and more particularly to a technology pertaining to diagnosis of abnormality of an NOX storage reduction (NSR) catalyst.
Description of the Related Art
An NSR catalyst is known as an exhaust gas purification device for a lean-burn internal combustion engine. The NSR catalyst stores NOX in the exhaust gas when the air-fuel ratio of the exhaust gas is a lean air-fuel ratio higher than the theoretical air-fuel ratio and desorbs and reduces NOX stored therein when the air-fuel ratio of the exhaust gas is a rich air-fuel ratio lower than the theoretical air fuel ratio. One known method of diagnosing an abnormality such as deterioration or failure of such an exhaust gas purification device is measuring the amount of NOX stored in the state in which the NOX storage capacity of the NSR catalyst is saturated (which will be hereinafter referred to as the saturation storage amount) and making a diagnosis that the exhaust gas purification device is in an abnormal condition if the saturation storage amount is smaller than a predetermined threshold (see, for example, patent literature 1).